Frames for sports rackets, and particularly for tennis rackets, present an engineering challenge. They must be strong enough to withstand enormous loads, be as nearly rigid as possible, and yet use only a few ounces of material. For example, a conventional tennis racket weighs approximately 12.5 to 14.5 ounces; and its center of gravity is in the vicinity of its throat, which makes the weight attributed to the frame extending around the ball-hitting region from 6 to 7 ounces. These few ounces of material must sustain a tremendous string load of up to 80 pounds per string and a ball-hitting load of 100 pounds or more, repeated for perhaps 40,000 shots without a failure. Understandably, sports racket frames have not yet fully met such a challenge.
Steel frame rackets are known to be too flexible or "whippy". Since steel is heavy, its walls have to be made thin to remain light in weight, giving its frame section insufficient moment of inertia for resisting bending and torsion loads.
Frame sections formed of aluminum alloy can have thicker walls and be more rigid, but they tend to permanently deform due to lower yield strength. Alcoa heat-treatable 60-T6 series or 70 series improve the strength of aluminum considerably, but not enough to eliminate frame problems.
Graphite and composite materials, although expensive, have produced frame strips of very high strength-to-weight ratios that increase possible alternatives.
Frame strips presently used in metal rackets fall into two categories--oval or rectangular tubular section and I-beam section with solid or tubular flanges. For the latter, the tubular flange on both ends of the web provides torsional and bending rigidity resisting ball impact; and the thick web provides a bearing seat supporting string holes. Although quite popular, the I-beam section has the inherent problem of a marginal moment of inertia to resist the pulling load from the strings in the plane of the string surface, since most of the sectional mass is along the longitudinal axis of the frame to provide a solid seating for the strings. For example, the moment-of-inertia ratio between the axis perpendicular to the web and the axis coinciding with the web for the HEAD EDGE racket frame section is 7.6 to 1.0.
For a rectangular tubular frame section, the disparity between moments of inertia along the two principal axes is not as drastic as for I-beam type frames, but even these are usually narrowed in the middle of the section to provide the necessary string support. Graphite rackets also follow the general geometry of metal tubing frames, and they too have a narrow neck where the string hole is bored through the frame strip.
I have thoroughly studied the problems of sports racket frames, and tennis racket frames in particular, and have used the finite-element structural mechanical analysis method to study the loads imposed on a tennis racket from the strings and from the impact of the ball. Through such analysis, I have discovered a better cross-sectional shape for a racket frame having several important advantages. My analysis not only revealed the weaknesses of conventional racket frames, but showed that frames having an improved cross-sectional shape can be made stronger and more rigid without increasing weight, even though still using existing materials.
Another important advantage of my improved frame section is a longer free vibrational length for the strings which substantially improves the performance of the string network. By keeping the free vibrational length of the strings to a maximum within the overall size limitations of a particular racket frame and by making the frame stronger and more rigid, my invention adds considerably to the performance of racket frames.